This is a 371 of PCT/EP99/07308 filed Oct. 1, 1999.
The present invention relates to hydrophilic, highly swellable hydrogels having high absorbency for water and aqeuous fluids, to a process for their preparation and to the use of these hydrogels.
Hydrophilic hydrogels obtainable by polymerization of unsaturated acids, for example acrylic acid, methacrylic acid, acrylamidopropanesulfonic acid, etc., in the presence of small amounts of multiply olefinically unsaturated compounds are already known as superabsorbent polymers. Also known are hydrophilic hydrogels obtainable by graft copolymerization of the olefinically unsaturated acids onto different matrices, for example polysaccharides, polyalkylene oxides and derivatives thereof.
The hydrogels mentioned are notable for high absorbency for water and aqueous solutions and are therefore widely used as absorbents in hygiene articles.
Such water-swellable hydrophilic polymers are generally prepared by free-radical polymerization in an aqueous solution which contains the monomers with or without a grafting base and crosslinker.
The water-swellable hydrophilic polymers produced for use in the hygiene and sanitary sector have a degree of neutralization in the range from 50 to 85 mol % based on the polymerized acid-functional monomer units, so that the hydrogels formed in use are pH neutral with regard to the skin.
The degree of neutralization is generally set prior to the polymerization, since this avoids the technically difficult neutralization of an acidic hydrogel of high viscosity. However, the polymerization of, for example, acrylic acid in the neutral pH range is slower, and leads to lower molecular weights, than the polymerization in the acidic range. This is explained by the electrostatic repellency between the most recently incorporated monomer unit and the next monomer unit to be incorporated, which repellency arises only minimally, if at all, in the case of a polymerization in the acidic pH range, since the monomer units are present in the uncharged, acidic form.
The trend toward ever thinner diaper constructions requires water-swellable hydrophilic polymers providing better and better performance characteristics with regard to absorption capacity, fluid acquisition and fluid transportation within the hygiene article, especially under a confining load.
EP-A-0 640 330, WO 95/22358, WO 95/26209 and WO 97/12575 describe a test for measuring the gel permeability of swollen hydrogel particles (Saline Flow Conductivity, SFC). This test determines the flow of a sodium chloride solution through a preswollen hydrogel particle layer under a pressure of 0.3 psi. Since, in this test method, the sodium chloride solution will flow through the swollen gel layer in the direction of the force of gravity, this method is less suitable for characterizing any capillary forces. In practice, however, it is relevant for the quality of hygiene articles that fluid can be transported under load even in a direction opposite to the force of gravity, i.e., capillary forces do play an important part.
It is an object of the present invention to provide hydrophilic, highly swellable hydrogels having improved absorption properties, especially improved capillarity under load, and a process for the production thereof.
We have found that this object is achieved by a hydrophilic, highly swellable hydrogel based on (co)polymerized monomers or based on graft (co)polymers obtainable by
a) free-radically (co)polymerizing one or more hydrophilic monomers or graft (co)polymerizing one or more hydrophilic monomers onto a grafting base, the average degree of neutralization of the acid-functional monomers being from 0 to 40 mol %,
b) comminuting the acidic hydrogel,
c) neutralizating the acidic hydrogel to an ultimate degree of neutralization of 50-85 mol % by adding a neutralizing agent,
d) drying, grinding and sieving the hydrogel particles characterized by
a centrifuge retention of at least 29 g/g for a 0.9% aqueous NaCl solution and
a vertical absorption (1 g) swell height of at least 3.5 cm coupled with an extractables content (16 h value) of less than 5% or
a centrifuge retention of at least 23 g/g for a 0.9% aqueous NaCl solution and
a vertical absorption (3 g) swell height of at least 5 cm
an extractables content (16 h value) of less than 4%.
The hydrophilic, highly swellable hydrogels of the invention and also the process for their production will now be more particularly described.
Hydrophilic monomers useful for preparing the water-swellable hydrophilic polymers of the invention include for example acids capable of addition polymerization, such as acrylic acid, methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, styrenesulfonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido-2-methylpropanephosphonic acid and their amides, hydroxyalkyl esters, amino- or ammonio-functional esters and amides. Water-soluble N-vinylamides or else diallyldimethylammonium chloride are also suitable.
Preferred hydrophilic monomers are compounds of the general formula (I) 
where
R1 is hydrogen, methyl or ethyl,
R2 is xe2x80x94COOR4, hydroxysulfonyl, phosphonyl, a (C1-C4)-alkanol-esterified phosphonyl group or a group of the general formula (II) 
R3 is hydrogen, methyl, ethyl or carboxyl,
R4 is hydrogen, amino-(C1-C4)-alkyl or hydroxy-(C1-C4)-alkyl, and
R5 is hydroxysulfonyl, phosphonyl or carboxyl.
Examples of (C1-C4)-alkanols are methanol, ethanol, n-propanol and n-butanol.
Particularly preferred hydrophilic monomers are acrylic acid and methacrylic acid.
When the monomers used are acids, their alkali metal or ammonium salts may be used as comonomers in a fraction of up to 40% by weight.
Useful grafting bases may be of natural or synthetic origin. Examples are starch, cellulose or cellulose derivatives and also other polysaccharides and oligosaccharides, polyvinyl alcohol, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides, and also hydrophilic polyesters. Useful polyalkylene oxides have for example the formula (III) 
where
R6 and R7 are independently hydrogen, alkyl, alkenyl or aryl,
X is hydrogen or methyl, and
n is an integer from 1 to 10000.
R6 and R7 are each for example linear or branched (C1-C10)-alkyl such as methyl, ethyl, propyl, isopropyl, n-butyl, (C2-C6)-alkenyl or aryl such as unsubstituted or (C1-C4)-alkyl-substituted phenyl.
R6 and R7 are each preferably hydrogen, (C1-C4)-alkyl, (C2-C6)-alkenyl or phenyl.
The hydrophilic, highly swellable hydrogels are preferably in a crosslinked state, i.e., they contain units polymerized into the polymer network that are derived from compounds having at least two double bonds.
Useful crosslinkers include in particular methylenebisacrylamide, methylenebismethacrylamide, esters of unsaturated mono- or polycarboxylic acids with polyols, such as diacrylate or triacrylate, e.g., butanediol diacrylate, butanediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate and also trimethylolpropane triacrylate, allyl compounds such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, pentaerythritol triallyl esters or allyl esters of phosphoric acid and also vinyl compounds such as vinyl acrylate, divinyl adipate, divinylbenzene and vinylphosphonic acid derivatives, as described for example in EP-A-0 343 427.
The polymerization may be initiated using high energy electromagnetic radiation or the customary chemical polymerization initiators, for example organic peroxides such as benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketone peroxide, cumene hydroperoxide, azo compounds such as azodiisobutyronitrile and also inorganic peroxy compounds such as ammonium persulfate, potassium persulfate or hydrogen peroxide, with or without reducing agents such as sodium bisulfite, and iron(II) sulfate or redox systems where the reducing component is an aliphatic or aromatic sulfinic acid, such as benzenesulfinic acid or toluenesulfinic acid or derivatives thereof, for example Mannich adducts of sulfinic acid, aldehydes and amino compounds.
Polymerization in aqueous solution is preferably conducted as a gel polymerization by utilizing the Trommsdorff-Norrish effect. It is particularly preferable for the polymerization to be carried out in the quiescent state without mechanical mixing, so that the hydrogel that forms is not exposed to any mechanical shearing forces which would raise the level of extractables. The polymerization may here be carried out not only batchwise, for example in a cylindrical reactor, but also continuously, for example by polymerization on a belt reactor.
The resultant hydrogels are coarsely comminuted by means of customary pulling and/or cutting tools, for example by the action of a discharging pump in the case of a polymerization in a cylindrical reactor or by a cutting roll or cutting roll combination in the case of a belt polymerization.
This provides provides acidic hydrogel particles which have to be adjusted to the desired ultimate degree of neutralization of 50-85 mol % based on acid-functional monomer units by neutralizing these acid-functional monomer units. This neutralization is a process which is technically difficult to carry out and which has to meet particular requirements. First, the gel must not be excessively sheared during the contacting with the neutralizing agent so as to avoid increasing the extractables content, which would have an adverse effect on the properties of the end product and accordingly is undesirable, and, secondly, neutralization has to be completely homogeneous.
The subsequent neutralization of acidic hydrogels is known in principle.
DE-A-26 12 846 discloses a process for preparing a water-absorbing resin by polymerizing at least one starch and/or cellulose with at least one water-soluble monomer having a polymerizable double bond and with a crosslinker. The polymers obtained are neutralized with alkalis, although the method of neutralization is not more particularly specified.
According to EP-A-0 205 674, acidic polymers are prepared at from 0 to 100xc2x0 C., preferably from 5 to 40xc2x0 C., and their pH is adjusted by subsequent partial neutralization of the hydrogel. Neutralization is effected here by adding the gel to a very dilute sodium hydroxide solution. This method is disadvantageous, since large amounts of water have to be evaporated at the drying stage owing to the very dilute nature of the sodium hydroxide solution.
EP-A-0 303 440 describes the production of a hydrated crosslinked gel polymer which has 10 to 50 mol % of the acid-functional monomers neutralized and which is adjusted to the desired ultimate degree of neutralization by adding a neutralizing agent in a reaction vessel having a plurality of rotary shafts each fitted with stirring blades. True, this process provides homogeneous neutralization, since new surfaces are constantly being generated for the gel particles, but the shearing force on the gel is too high and leads to an undesirable increase in extractables.
EP-A-0 238 050 claims a process for the batchwise production of finely divided crosslinked water-absorbing polymers by conducting the polymerization in a kneader and having a degree of neutralization for the (meth)acrylic acid in the range from 0 to 100 mol %. The polymerization batch is neutralized to the desired ultimate pH in the kneader used for the polymerization, either during the polymerization or subsequently thereto. This again provides homogeneous neutralization, but the shearing forces applied are too high, so that an undesirable increase in the extractables content occurs.
In U.S. Pat. No. 5,453,323 and EP-A-0 530 438, acrylic acid is used together with water-soluble hydroxyl-containing polymers to prepare under adiabatic conditions and without neutralization of the monomers polymer gels which are subsequently comminuted in an unspecified meat grinder. The neutralizing agent is added to this comminuted gel and the mixture is again chopped. The postcrosslinker is then added and the gel is again chopped three times in order that all the reactants may be incorporated in the gel in a homogeneous manner. This repeated chopping of the gel exerts an undesirable shearing stress on the gel, elevating the level of extractables.
EP-A-0 629 411 describes the polymerization of acrylic acid with crosslinkers. The gel obtained is subsequently partially neutralized with an alkali metal salt and further crosslinked by addition of a crosslinker. The method of neutralization is not further specified in the reference; one example mentions kneading the gel with the neutralizing agent in an extruder.
DE-A-195 29 348 describes preparing superabsorbent polymers by polymerizing a partially preneutralized monomer solution under adiabatic conditions. The degree of preneutralization of the acid-functional monomers is in the range from 5 to 30 mol %. The acidic gel is neutralized after its comminution in simple mixing assemblies such as a rotating drum or in a Drais mixer, the aqueous solution of the bases being introduced via nozzles or spray injectors, for example. True, this avoids any mechanical damage to the polymer gel, but cannot provide homogeneous neutralization, since the gel is not destructured in the course of the mixing with the neutralizing agent. The pH inhomogeneities of the gel in turn lead to inferior drying, which is undesirable for economic reasons.
The acidic hydrogel is preferably neutralized by destructuring and mixing it together with the neutralizing agent in a mincer comprising a system ofscrew, rotating blade, restricted flow zone and breaker plate and providing a power output of from 1,000 to 6,000 Wh/m3, preferably of from 2,500 to 5,000 Wh/m3, by passing the hydrogel through a zone having an energy dissipation density of from 400 to 800 W/l of mixing volume. The process utilizes residence times of from 5 to 30 seconds. The frequency of the rotating blade is 1-5 sxe2x88x921, preferably 3-4 sxe2x88x921. To reduce the shearing forces on mixing in the restricted flow region above the breaker plate of the apparatus, the capillaries in the breaker plate are conical. The open area of the breaker plate is from 20 to 40%, preferably from 25 to 35%, and the initial hole diameter is from 4 to 16 mm, preferably from 8 to 10 mm, coupled with a conical widening at an angle of from 8xc2x0 to 20xc2x0, preferably from 10xc2x0 to 15xc2x0. A mincer is similar in equipment terms to an extruder, but-exerts less shearing force.
The design described provides a combination of high mixing efficiency and of benign mechanical treatment of the mixture of hydrogel and neutralizing agent. A single-stage treatment would prove to be absolutely adequate for homogeneous distribution, avoiding the repeated mincing of the gel which would in turn lead to an undesirable increase in the shearing stress on the gel.
The choice of neutralizing agent is not critical, suitable neutralizing agents being alkali metal hydroxides, ammonia, aliphatic primary and secondary amines, alkali metal carbonates and alkali metal bicarbonates. Particular preference is given to sodium hydroxide and sodium carbonate. The neutralizing agent may be added in liquid form, for example aqueous sodium hydroxide solution, in solid form, for example sodium carbonate powder, or in gaseous form, for example ammonia.
The specific design of the mincer also makes it possible to mix other reactants or materials with the polymer gel to be neutralized according to the invention. This avoids the repeated mincing of the gel which would in turn lead to an undesirable increase in the shearing stress on the gel.
For instance, the gel may be admixed with reactants capable of reacting with free acrylic acid, for example amino acids such as cysteine or lysine, hydroxylamine and/or its salts such as hydrochloride or sulfate, hydrazine and/or its salts, ozone or sulfur compounds having a reducing effect, such as alkali metal sulfites, bisulfites or disulfites, sodium thiosulfate or mercapto compounds.
The gel may also be admixed with materials capable of reacting with the carboxyl groups of the hydrogel by crosslinking. Examples of such materials are polyhydric alcohols, polyacid amines, polyamidoamines and their reaction products with epichlorohydrin, di- and polyepoxides, bis- and polyaziridines, bis- and polyoxazolines, di- and polyisocyanates, ethylene carbonate or oxazolidone.
It is further possible in this stage to mix the gel with fines of superabsorbent polymers that are obtained, for example, from the production of water-swellable hydrophilic hydrogels during the grinding and subsequent classification of the dried hydrogels.
The acidic hydrogel may also be neutralized according to a two-step process. In this process, the first step comprises setting a degree of neutralization of not less than 50% by weight, preferably not less than 55% by weight, particularly preferably not less than 60% by weight, in a mincer which meets the above-described conditions. The second step raises the degree of neutralization to the desired ultimate degree of neutralization by spraying the hydrogel particles with a neutralizing agent or its aqueous solution, i.e., without mechanical shearing of the gel particles. The neutralizing agent in the second step may be identical or different from the neutralizing agent in the first step. The second neutralizing step is preferably carried out using aqueous sodium hydroxide solution.
Various ways are known for drying hydrogel particles. For instance, they may be dried by the thin film drying process, for example by means of a biaxial can dryer; by the plate drying process, whereby the hydrogel polymer particles are loaded onto plates in several layers in a drying chamber in which hot air circulates; by the rotating drum process using can dryers; or by the conveyor belt process, hereinbelow also referred to as simply belt drying. Drying is preferably carried out with can dryers or by the operations of belt drying, where foraminous trays of a circle conveyor are loaded in a tunnel with the material to be dried and the material is dried by blowing hot air through the tray holes during the passage through the tunnel.
The dried hydrogel is precomminuted where appropriate and then ground, preferably by means of a roll mill in order that the production of fines may be minimized. Sieving is carried out subsequently to set the particle size distribution, which is generally in the range from 100 to 1000 xcexcm, preferably from 120 to 850 xcexcm. Oversize particles may be resubmitted to grinding, while undersize particles may be recycled back into the production process, for example by mixing with the gel to be neutralized in the postneutralization step in the mincer, or be used for distinct purposes.
In a preferred embodiment of the invention, the absorption properties of the hydrophilic, highly swellable hydrogels thus obtained are still further improved by a subsequent surface postcrosslinking step. In this step, compounds capable of reacting with the carboxyl groups of the hydrogel by crosslinking are applied to the surface of the hydrogel particles, preferably in the form of an aqueous solution. Useful postcrosslinking agents include for example di- or polyglycidyl compounds such as phosphonyl diglycidyl ether or ethylene glycol diglycidyl ether, alkoxysilyl compounds, polyaziridines, polyamines or polyamidoamines and also their rection products with epichlorohydrin, polyols such as ethylene glycol, 1,2-propanediol, 1,4-butanediol, glycerol, di- and polyglycerol, pentaerythritol, sorbitol, the ethoxylates of these polyols and their esters with carboxylic acids or carbonic acid, ethylene carbonate, propylene carbonate, oxazolidone, bisoxazoline, polyoxazolines, di- and polyisocyanates. If necessary, acidic catalysts such as, for example, p-toluenesulfonic acid, phosphoric acid, boric acid or ammonium dihydrogenphosphate can be added.
Suitable mixing assemblies for spraying the hydrogel particles with crosslinker solution include for example Patterson-Kelly mixers, DRAIS turbulence mixers, Lbdige mixers, screw mixers, plate mixers, fluidized bed mixers, Schugi mixers. The spraying of the crosslinker solution may be followed by a temperature treatment step, preferably in a downstream dryer, at from 80 to 230xc2x0 C., preferably 80-190xc2x0 C., particularly preferably from 100 to 160xc2x0 C., for from 5 minutes to 6 hours, preferably from 10 minutes to 2 hours, particularly preferably form 10 minutes to 1 hour; lysis products may be removed as well as solvent fractions.
In a particularly preferred embodiment of the invention the hydrophilicity of the hydrogel particle surface is additionally modified through formation of metal complexes. The formation of metal complexes on the outer shell of the hydrogel particles is effected by spraying with solutions of divalent or more highly valent metal salt solutions to allow-the-metal cations to react with the carboxyl groups of hydrogel to form complexes. Examples of di- or more highly valent metal cations are Mg2+, Ca2+, Al3+, Sc3+, Ti4+, Mn2+, Fe2+/3+, Co2+, Ni2+, Cu+/2+, Zn2+, Y3+, Zr4+, Ag+, La3+, Ce4+, Hf4+, and Au+/3+, preferred metal cations being Mg2+, Ca2+, Al3+, Ti4+, Zr4+ and La3+; particularly preferred metal cations are Al3+, Ti4+ and Zr4+. Metal cations may be used alone or else as a mixture with each or one another. Of the metal cations mentioned, any metal salt possessing sufficient solubility in the solvent to be used is suitable. Metal salts with weakly complexing anions, for example, chloride, nitrate or sulfate, are particularly suitable. Useful solvents for the metal salts include water, alcohols, DMF, DMSO and mixtures thereof. Particular preference is given to water and water/alcohol mixtures, for example water/methanol or water/1,2-propanediol.
The spraying of the metal salt solution onto the hydrogel particles may take place both before and after the surface postcrosslinking of hydrogel particles. In a particularly preferred process, the spraying on of the metal salt solution takes place in the same step as the spraying on of the crosslinker solution, the two solutions being sprayed on separately in succession or simultaneously via two nozzles, or crosslinker solution and metal salt solution may be sprayed on together via a single nozzle.
If desired, the hydrogel particles may be further modified by admixture of finely divided inorganic solids, for example silica, alumina, titania and iron(II) oxide to further augment the effects of the surface aftertreatment. Particular preference is given to the admixture of hydrophilic silica or of alumina having an average primary particle size of from 4 to 50 nm and a specific surface area of 50-450 m2/g. The admixture of finely divided inorganic solids preferably takes place after the surface modification through crosslinking/complexing, but may also be carried out before or during these surface modifications.
Hydrogels of the invention are notable for outstanding absorbency, high capillarity coupled with high gel strength and low levels of extractables and are therefore very useful as absorbents for water and aqueous fluids, especially body fluids, for example urine or blood, for example in hygiene articles such as, for example, infant and adult diapers, sanitary napkins, tampons and the like. But they may also be used as soil improvers in agriculture and market gardening, as moisture binders in cable sheathing and also for thickening aqueous wastes.
Description of test methods used in examples:
CRC (Centrifuge Retention Capacity)
0.2 g of hydrogel (particle size fraction 106-850 xcexcm) is weighed into a teabag 60xc3x9785 mm in size, which is subsequently welded shut. The teabag is then placed in an excess of 0.9% by weight sodium chloride solution (at least 0.83 1 of sodium chloride solution/1 g of hydrogel). After a swelling time of 30 minutes, the teabag is removed from the sodium chloride solution and centrifuged at 250 g for three minutes. The centrifuged teabag is weighed to determine the amount of liquid retained by the hydrogel.
Extractables (16 h)
1 g of hydrogel (particle size fraction 106-850 xcexcm) is stirred into 200 ml of 0.9% by weight sodium chloride solution. The beaker is sealed and the mixture is stirred for 16 h. This is followed by filtration through a 0.22 xcexcm filter and determination of the level of extractables by an acid-base titration of the carboxyl groups (titration with 0.1 normal NaOH to pH 10, then with 0.1 normal HCl to pH 2.7).
Vertical Absorption
The capillarity is determined with the aid of vertical absorption. The test apparatus consists of measuring cells and a liquid container. The measuring cells constitute a cylindrical Plexiglas tube 2.6 cm in internal diameter and 15 cm in length. The upper end of the tube is open, the lower end possesses a 36 xcexcm sieve bottom. At a height of 3 cm (from the lower end of the tube) the tube possesses a supporting ring. The liquid container is a Plexiglas box 30.0 cm in length, 20.5 cm in width and 3.8 cm in height. At a distance of 1.5 cm to one transverse side an overflow wall 2 cm in height has been fitted. On the opposite side there is a connection to the liquid container, so that a constant level of liquid is ensured. The Plexiglas box has a removable lid which is provided with 6 circularly round holes each 3.2 cm in diameter. To perform the measurement, 1 g of hydrogel (vertical absorption 1 g) or 3 g of hydrogel (vertical absorption 3 g) is or are weighed into a measuring cell, and the hydrogel particles are uniformly distributed over the sieve bottom. The hydrogel particles are then covered with a close clearance Plexiglas disk and a close clearance Plexiglas cylinder with metal rod is introduced, the total weight of the Plexiglas disk and of the cylinder with rod being 100 g, so that the hydrogel particles are under a pressure of 19.6 g/cm2. The liquid container is filled with 0.9% by weight sodium chloride solution. The measuring cell is then dipped into the liquid (to a depth of 1.2 cm) through a hole in the lid while the measuring cell is held by the supporting ring. Up to 6 measuring cells can be measured at any one time. The measuring cells are left in the liquid container for 60 minutes, during which the hydrogel particles swell and increase in weight by absorbing liquid against the force of gravity. Owing to the very high surface coverage due to hydrogel particles, a high gel layer permeability and a high capillarity are needed to obtain a high swell height. After 60 minutes, the measuring cell is removed from the liquid container and the height of the swollen gel is determined.